Optical device wafer processing method

ABSTRACT

An optical device layer (ODL) in an optical device wafer is transferred to a transfer substrate. The ODL is formed on the front side of an epitaxy substrate through a buffer layer. The ODL is partitioned by a plurality of crossing streets to define regions where a plurality of optical devices are formed. The transfer substrate is bonded to the front side of the ODL, and the epitaxy substrate is cut along crossing streets into a plurality of blocks. A laser beam is applied to the epitaxy substrate from the back side of the epitaxy substrate to the unit of the optical device wafer and the transfer substrate in the condition where the focal point of the laser beam is set in the buffer layer, thereby decomposing the buffer layer. The epitaxy substrate divided into the plurality of blocks is peeled off from the ODL.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical device wafer processing method for transferring an optical device layer in an optical device wafer to a transfer substrate, the optical device layer being composed of an n-type semiconductor layer and a p-type semiconductor layer, formed on the front side of an epitaxy substrate such as a sapphire substrate and a silicon carbide substrate through a buffer layer, and partitioned by a plurality of crossing streets to define a plurality of regions where optical devices such as light emitting diodes and laser diodes are respectively formed.

2. Description of the Related Art

In an optical device fabrication process, an optical device layer composed of an n-type semiconductor layer and a p-type semiconductor layer is formed on the front side of a substantially disk-shaped epitaxy substrate such as a sapphire substrate and a silicon carbide substrate through a buffer layer, and this optical device layer is partitioned by a plurality of crossing streets into a plurality of regions where a plurality of optical devices such as light emitting diodes and laser diodes are respectively formed, thus constituting an optical device wafer. The optical device wafer is divided along the streets to thereby obtain the individual optical devices (see Japanese Patent Laid-open No. Hei 10-305420, for example).

Further, as a technique for improving the luminance of an optical device, a manufacturing method called lift-off is disclosed in JP-T-2005-516415. In an optical device wafer, an optical device layer composed of an n-type semiconductor layer and a p-type semiconductor layer is formed on the front side of an epitaxy substrate such as a sapphire substrate and a silicon carbide substrate through a buffer layer. The above-mentioned method called lift-off includes the steps of bonding the optical device layer of the optical device wafer through a bonding metal layer formed of gold (Au), platinum (Pt), chromium (Cr), indium (In), or palladium (Pd) to a transfer substrate formed of molybdenum (Mo), copper (Cu), or silicon (Si) and next applying a laser beam from the back side of the epitaxy substrate to the buffer layer to thereby peel off the epitaxy substrate, thus transferring the optical device layer to the transfer substrate.

SUMMARY OF THE INVENTION

In the technique disclosed in JP-T-2005-516415, the step of bonding the transfer substrate to the optical device layer formed on the front side of the epitaxy substrate is performed by heating to 220 to 300° C. Accordingly, warpage occurs in the unit of the epitaxy substrate and the transfer substrate bonded together because of a difference in coefficient of linear expansion between the epitaxy substrate and the transfer substrate. As a result, in peeling off the epitaxy substrate from the optical device layer, it is difficult to accurately set the focal point of the laser beam in the buffer layer formed between the epitaxy substrate and the optical device layer, causing such a problem that the optical device layer may be damaged or the buffer layer may not be surely decomposed to result in unsmooth peeling of the epitaxy substrate.

It is therefore an object of the present invention to provide an optical device wafer processing method which can smoothly transfer the optical device layer formed on the front side of the epitaxy substrate constituting the optical device wafer to the transfer substrate without damage to the optical device layer.

In accordance with an aspect of the present invention, there is provided an optical device wafer processing method for transferring an optical device layer in an optical device wafer to a transfer substrate, the optical device layer being formed on the front side of an epitaxy substrate through a buffer layer, the optical device layer being partitioned by a plurality of crossing streets to define a plurality of regions where a plurality of optical devices are respectively formed, the optical device wafer processing method including a transfer substrate bonding step of bonding the transfer substrate to the front side of the optical device layer formed on the front side of the epitaxy substrate through the buffer layer; an epitaxy substrate dividing step of cutting the epitaxy substrate along predetermined ones of the plurality of crossing streets into a plurality of blocks after performing the transfer substrate bonding step; a peeling laser beam applying step of applying a laser beam having a transmission wavelength to the epitaxy substrate from the back side of the epitaxy substrate to a unit of the optical device wafer and the transfer substrate in a condition where the focal point of the laser beam is set in the buffer layer after performing the epitaxy substrate dividing step, thereby decomposing the buffer layer; and an epitaxy substrate peeling step of peeling off the epitaxy substrate divided into the plurality of blocks from the optical device layer after performing the peeling laser beam applying step.

Preferably, the epitaxy substrate dividing step includes the step of cutting the epitaxy substrate along the predetermined streets by using a cutting blade. Alternatively, the epitaxy substrate dividing step includes the step of cutting the epitaxy substrate along the predetermined streets by applying a laser beam to the epitaxy substrate along the predetermined streets. Preferably, the optical device wafer processing method further includes a transfer substrate dividing step of cutting the transfer substrate together with the optical device layer along the plurality of crossing streets after performing the epitaxy substrate peeling step.

In the epitaxy substrate dividing step of the optical device wafer processing method according to the present invention, the epitaxy substrate is cut along the predetermined streets into the plural blocks to thereby relieve the warpage occurring in the unit of the epitaxy substrate and the transfer substrate due to the difference in coefficient of linear expansion between the epitaxy substrate and the transfer substrate. Accordingly, in performing the peeling laser beam applying step, the focal point of the laser beam can be accurately positioned in the buffer layer. Accordingly, it is possible to eliminate the problem such that the focal point of the laser beam may be set in the optical device layer to cause damage to the optical device layer. The buffer layer is formed of gallium nitride (GaN), and it is decomposed as 2GaN→2Ga+N2 by the application of the laser beam. Thus, N2 gas is produced to have an adverse effect on the optical device layer. However, since the epitaxy substrate is divided along division grooves into the plural blocks, the N2 gas produced can be relieved through the division grooves to thereby reduce the adverse effect on the optical device layer.

The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of an optical device wafer to be processed by the optical device wafer processing method according to the present invention;

FIG. 1B is an enlarged sectional view of an essential part of the optical device wafer shown in FIG. 1A;

FIG. 2A is a perspective view for illustrating a transfer substrate bonding step in the optical device wafer processing method according to the present invention;

FIG. 2B is an enlarged sectional view of an essential part of the unit of the optical device wafer and the transfer substrate obtained by the transfer substrate bonding step shown in FIG. 2A;

FIG. 3 is a perspective view of an essential part of a cutting apparatus for performing a first preferred embodiment of an epitaxy substrate dividing step in the optical device wafer processing method according to the present invention;

FIGS. 4A and 4B are sectional side views for illustrating the first preferred embodiment of the epitaxy substrate dividing step in the optical device wafer processing method according to the present invention;

FIG. 4C is an enlarged sectional view of an essential part of the unit of the optical device wafer and the transfer substrate obtained by the first preferred embodiment of the epitaxy substrate dividing step shown in FIGS. 4A and 4B;

FIG. 4D is a perspective view of the unit of the optical device wafer and the transfer substrate obtained by the first preferred embodiment of the epitaxy substrate dividing step shown in FIGS. 4A and 4B;

FIG. 5 is a perspective view of an essential part of a laser processing apparatus for performing a second preferred embodiment of the epitaxy substrate dividing step in the optical device wafer processing method according to the present invention;

FIGS. 6A and 6B are sectional side views for illustrating the second preferred embodiment of the epitaxy substrate dividing step in the optical device wafer processing method according to the present invention;

FIG. 6C is an enlarged sectional view of an essential part of the unit of the optical device wafer and the transfer substrate obtained by the second preferred embodiment of the epitaxy substrate dividing step shown in FIGS. 6A and 6B;

FIG. 6D is a perspective view of the unit of the optical device wafer and the transfer substrate obtained by the second preferred embodiment of the epitaxy substrate dividing step shown in FIGS. 6A and 6B;

FIG. 7 is a perspective view for illustrating an optical device wafer supporting step in the optical device wafer processing method according to the present invention;

FIG. 8 is a perspective view of an essential part of a laser processing apparatus for performing a peeling laser beam applying step in the optical device wafer processing method according to the present invention;

FIGS. 9A to 9C are views for illustrating the peeling laser beam applying step in the optical device wafer processing method according to the present invention;

FIG. 10 is a perspective view for illustrating an epitaxy substrate peeling step in the optical device wafer processing method according to the present invention;

FIG. 11 is a perspective view of an essential part of a cutting apparatus for performing a first preferred embodiment of a transfer substrate dividing step in the optical device wafer processing method according to the present invention;

FIGS. 12A and 12B are sectional side views for illustrating the first preferred embodiment of the transfer substrate dividing step in the optical device wafer processing method according to the present invention;

FIG. 12C is an enlarged sectional view of an essential part of the unit of the optical device wafer and the transfer substrate obtained by the first preferred embodiment of the transfer substrate dividing step shown in FIGS. 12A and 12B;

FIG. 12D is a perspective view of an optical device wafer having a crossing division groove formed along a plurality of crossing streets by the transfer substrate dividing step shown in FIGS. 12A and 12B;

FIG. 13 is a perspective view of an essential part of a laser processing apparatus for performing a second preferred embodiment of the transfer substrate dividing step in the optical device wafer processing method according to the present invention;

FIGS. 14A and 14B are sectional side views for illustrating the second preferred embodiment of the transfer substrate dividing step in the optical device wafer processing method according to the present invention;

FIG. 14C is an enlarged sectional view of an essential part of the unit of the optical device wafer and the transfer substrate obtained by the second preferred embodiment of the transfer substrate dividing step shown in FIGS. 14A and 14B; and

FIG. 14D is a perspective view of the unit of the optical device wafer and the transfer substrate obtained by the second preferred embodiment of the transfer substrate dividing step shown in FIGS. 14A and 14B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the optical device wafer processing method according to the present invention will now be described in detail with reference to the attached drawings. FIG. 1A is a perspective view of an optical device wafer 2 to be processed by the optical device wafer processing method according to the present invention, and FIG. 1B is an enlarged sectional view of an essential part of the optical device wafer 2 shown in FIG. 1A. The optical device wafer 2 shown in FIGS. 1A and 1B is formed by epitaxial growth of an optical device layer 21 on the front side 20 a of a substantially circular epitaxy substrate 20 such as a sapphire substrate and a silicon carbide substrate. The optical device layer 21 is composed of an n-type gallium nitride semiconductor layer 211 and a p-type gallium nitride semiconductor layer 212. In forming the optical device layer 21 composed of the n-type gallium nitride semiconductor layer 211 and the p-type gallium nitride semiconductor layer 212 on the front side 20 a of the epitaxy substrate 20 by epitaxial growth, a buffer layer 22 of gallium nitride (GaN) is formed between the front side 20 a of the epitaxy substrate 20 and the n-type gallium nitride semiconductor layer 211 forming the optical device layer 21. In the thus configured optical device wafer 2, the epitaxy substrate 20 has a thickness of 430 μm, for example, and the optical device layer 21 has a thickness of 5 μm, for example, inclusive of the thickness of the buffer layer 22 in the embodiment shown in the figure. As shown in FIG. 1A, the optical device layer 21 is partitioned by a plurality of crossing streets 23 to define a plurality of regions where a plurality of optical devices 24 are respectively formed.

To peel off the epitaxy substrate 20 from the optical device layer 21 in the optical device wafer 2 and transfer the optical device layer 21 to a transfer substrate, a transfer substrate bonding step is first performed in such a manner that the transfer substrate is bonded to the front side 21 a of the optical device layer 21. More specifically, as shown in FIGS. 2A and 2B, a transfer substrate 3 having a thickness of 220 μm, for example, is bonded through a bonding metal layer 4 to the front side 21 a of the optical device layer 21 formed on the front side 20 a of the epitaxy substrate 20 constituting the optical device wafer 2. The transfer substrate 3 is formed of molybdenum (Mo), copper (Cu), or silicon (Si), for example. The bonding metal layer 4 is formed of gold (Au), platinum (Pt), chromium (Cr), indium (In), or palladium (Pd), for example, as a bonding metal.

The transfer substrate bonding step is performed in the following manner. The bonding metal mentioned above is deposited by evaporation on the front side 21 a of the optical device layer 21 formed on the front side 20 a of the epitaxy substrate 20 or on the front side 3 a of the transfer substrate 3, thereby forming the bonding metal layer 4 having a thickness of about 3 μm. Thereafter, the bonding metal layer 4 is brought into pressure contact with the front side 3 a of the transfer substrate 3 or the front side 21 a of the optical device 21, thereby bonding the front side 3 a of the transfer substrate 3 through the bonding metal layer 4 to the front side 21 a of the optical device layer 21 constituting the optical device wafer 2. In bonding the transfer substrate 3 to the front side 21 a of the optical device layer 21 formed on the front side 20 a of the epitaxy substrate 20, the epitaxy substrate 20 and the transfer substrate 3 are heated to 220 to 300° C. Accordingly, warpage occurs in the unit of the epitaxy substrate 20 and the transfer substrate 3 bonded together because of a difference in coefficient of linear expansion between the epitaxy substrate 20 and the transfer substrate 3. In the case that the diameter of the epitaxy substrate 20 is 10 cm, the amount of this warpage is about 0.5 mm.

After performing the transfer substrate bonding step, an epitaxy substrate dividing step is performed in such a manner that the epitaxy substrate 20 to which the transfer substrate 3 is bonded is cut along predetermined ones of the plural streets 23 into a plurality of blocks. A first preferred embodiment of the epitaxy substrate dividing step will now be described with reference to FIG. 3 and FIGS. 4A to 4D. The first preferred embodiment of the epitaxy substrate dividing step is performed by using a cutting apparatus 5 shown in FIG. 3. The cutting apparatus 5 shown in FIG. 3 includes a chuck table 51 for holding a workpiece, cutting means 52 having a cutting blade 521 for cutting the workpiece held on the chuck table 51, and imaging means 53 for imaging the workpiece held on the chuck table 51. The chuck table 51 is so configured as to hold the workpiece under suction. The chuck table 51 is movable in a feeding direction shown by an arrow X in FIG. 3 by feeding means (not shown) and also movable in an indexing direction shown by an arrow Y in FIG. 3 by indexing means (not shown).

The cutting blade 521 of the cutting means 52 is rotatable about an axis extending in the indexing direction and movable in a direction perpendicular to the upper surface of the chuck table 51. The imaging means 53 includes an ordinary imaging device (CCD) for imaging the workpiece by using visible light, infrared light applying means for applying infrared light to the workpiece, an optical system for capturing the infrared light applied to the workpiece by the infrared light applying means, and an imaging device (infrared CCD) for outputting an electrical signal corresponding to the infrared light captured by the optical system. An image signal output from the imaging means 53 is transmitted to control means (not shown).

The epitaxy substrate dividing step using the cutting apparatus 5 is performed in the following manner. The unit of the optical device wafer 2 and the transfer substrate 3 bonded together in the transfer substrate bonding step mentioned above is placed on the chuck table 51 of the cutting apparatus 5 in the condition where the transfer substrate 3 comes into contact with the upper surface of the chuck table 51 as shown in FIG. 3. Accordingly, the back side 20 b of the epitaxy substrate 20 constituting the optical device wafer 2 bonded to the transfer substrate 3 is oriented upward. By operating suction means (not shown), the unit of the optical device wafer 2 and the transfer substrate 3 is held under suction on the chuck table 51. The chuck table 51 thus holding the unit of the optical device wafer 2 and the transfer substrate 3 under suction is moved to a position directly below the imaging means 53 by the feeding means (not shown).

When the chuck table 51 is positioned directly below the imaging means 53, an alignment operation is performed by the imaging means 53 and the control means (not shown) to detect a cutting area of the epitaxy substrate 20 of the optical device wafer 2. The imaging means 53 and the control means (not shown) perform image processing such as pattern matching for making the alignment between the cutting blade 521 and the streets 23 extending in a first direction on the optical device layer 21 of the optical device wafer 2, thereby performing the alignment in the cutting area for the streets 23 extending in the first direction (alignment step). Similarly, the imaging means 53 and the control means perform the alignment in the cutting area for the other streets 23 extending in a second direction perpendicular to the first direction on the optical device layer 21 of the optical device wafer 2.

Although the optical device layer 21 where the streets 23 are formed is present below the epitaxy substrate 20, the streets 23 can be imaged through the epitaxy substrate 20 in the case that the epitaxy substrate 20 is formed from a transparent member such as a sapphire substrate. Further, even in the case that the epitaxy substrate 20 is not transparent, the streets 23 can be imaged through the epitaxy substrate 20 because the imaging means 53 includes the infrared light applying means for applying infrared light, the optical system for capturing the infrared light, and the imaging device (infrared CCD) for outputting an electrical signal corresponding to the infrared light as mentioned above.

After performing the alignment operation for detecting the cutting area of the epitaxy substrate 20 of the optical device wafer 2 bonded to the transfer substrate 3 held on the chuck table 51, the chuck table 51 holding the unit of the optical device wafer 2 and the transfer substrate 3 is moved to a cutting start position below the cutting blade 521 as shown in FIG. 4A. At this cutting start position, one end (left end as viewed in FIG. 4A) of a predetermined one of the streets 23 extending in the first direction is positioned on the right side of the cutting blade 521 by a predetermined small amount as shown in FIG. 4A. Thereafter, the cutting blade 521 is rotated in the direction shown by an arrow 521 a in FIG. 4A and simultaneously moved down by a predetermined amount in the direction shown by an arrow Z1 in FIG. 4A from a standby position shown by a phantom line in FIG. 4A to a working position shown by a solid line in FIG. 4A. This movement from the standby position to the working position is performed by operating vertically moving means (not shown) for moving the cutting blade 521 in the direction perpendicular to the upper surface of the chuck table 51. The working position of the cutting blade 521 is set so that the outer circumference of the cutting blade 521 reaches the buffer layer 22. After moving the cutting blade 521 from the standby position to the working position as mentioned above, the chuck table 51 is moved at a predetermined feed speed in the direction shown by an arrow X1 in FIG. 4A as rotating the cutting blade 521 in the direction shown by the arrow 521 a. When the other end (right end as viewed in FIG. 4B) of the predetermined street 23 extending in the first direction reaches a position on the left side of the cutting blade 521 by a predetermined small amount as shown in FIG. 4B, the movement of the chuck table 51 is stopped. Thereafter, the cutting blade 521 is raised to the standby position shown by a phantom line in FIG. 4B in the direction shown by an arrow Z2 in FIG. 4B.

As a result, the epitaxy substrate 20 constituting the optical device wafer 2 is cut along the predetermined street 23 extending in the first direction to form a division groove 201 as a cut groove as shown in FIG. 4C (epitaxy substrate dividing step). The epitaxy substrate dividing step is performed along predetermined ones of the plural crossing streets 23 extending in the first direction and the second direction perpendicular to the first direction to thereby divide the epitaxy substrate 20 into a plurality of blocks 200 as shown in FIG. 4D. As mentioned above, the unit of the epitaxy substrate 20 and the transfer substrate 3 has a warp of about 0.5 mm. This warp is relieved to some extent by the suction on the chuck table 51, but does not become zero. Accordingly, there is a case that the transfer substrate 3 may be cut by the cutting blade 521 along the predetermined streets 23.

A second preferred embodiment of the epitaxy substrate dividing step will now be described with reference to FIG. 5 and FIGS. 6A to 6D. The second preferred embodiment of the epitaxy substrate dividing step is performed by using a laser processing apparatus 6 shown in FIG. 5. The laser processing apparatus 6 shown in FIG. 5 includes a chuck table 61 for holding a workpiece, laser beam applying means 62 for applying a laser beam to the workpiece held on the chuck table 61, and imaging means 63 for imaging the workpiece held on the chuck table 61. The chuck table 61 is so configured as to hold the workpiece under suction. The chuck table 61 is movable in a feeding direction shown by an arrow X in FIG. 5 by feeding means (not shown) and also movable in an indexing direction shown by an arrow Y in FIG. 5 by indexing means (not shown).

The laser beam applying means 62 includes a cylindrical casing 621 extending in a substantially horizontal direction and focusing means 622 mounted on the front end of the casing 621 for applying a pulsed laser beam to the workpiece held on the chuck table 61. The imaging means 63 is mounted on the front end portion of the casing 621 constituting the laser beam applying means 62. The imaging means 63 includes an ordinary imaging device (CCD) for imaging the workpiece by using visible light, infrared light applying means for applying infrared light to the workpiece, an optical system for capturing the infrared light applied to the workpiece by the infrared light applying means, and an imaging device (infrared CCD) for outputting an electrical signal corresponding to the infrared light captured by the optical system. An image signal output from the imaging means 63 is transmitted to control means (not shown) to be described later.

The epitaxy substrate dividing step using the laser processing apparatus 6 is performed in the following manner. The unit of the optical device wafer 2 and the transfer substrate 3 bonded together in the transfer substrate bonding step mentioned above is placed on the chuck table 61 of the laser processing apparatus 6 in the condition where the transfer substrate 3 comes into contact with the upper surface of the chuck table 61 as shown in FIG. 5. Accordingly, the back side 20 b of the epitaxy substrate 20 constituting the optical device wafer 2 bonded to the transfer substrate 3 is oriented upward. By operating suction means (not shown), the unit of the optical device wafer 2 and the transfer substrate 3 is held under suction on the chuck table 61. The chuck table 61 thus holding the unit of the optical device wafer 2 and the transfer substrate 3 under suction is moved to a position directly below the imaging means 63 by the feeding means (not shown).

When the chuck table 61 is positioned directly below the imaging means 63, an alignment operation is performed by the imaging means 63 and the control means (not shown) to detect a processing area of the epitaxy substrate 20 to be laser-processed of the optical device wafer 2. The imaging means 63 and the control means (not shown) perform image processing such as pattern matching for making the alignment between the streets 23 extending in the first direction on the optical device layer 21 of the optical device wafer 2 and the focusing means 622 of the laser beam applying means 62 for applying a laser beam along the streets 23, thereby performing the alignment in the processing area for the streets 23 extending in the first direction (alignment step). Similarly, the imaging means 63 and the control means perform the alignment in the processing area for the other streets 23 extending in the second direction perpendicular to the first direction on the optical device layer 21 of the optical device wafer 2.

Although the optical device layer 21 where the streets 23 are formed is present below the epitaxy substrate 20, the streets 23 can be imaged through the epitaxy substrate 20 in the case that the epitaxy substrate 20 is formed from a transparent member such as a sapphire substrate. Further, even in the case that the epitaxy substrate 20 is not transparent, the streets 23 can be imaged through the epitaxy substrate 20 because the imaging means 63 includes the infrared light applying means for applying infrared light, the optical system for capturing the infrared light, and the imaging device (infrared CCD) for outputting an electrical signal corresponding to the infrared light as mentioned above.

After performing the alignment operation for detecting the processing area of the epitaxy substrate 20 of the optical device wafer 2 bonded to the transfer substrate 3 held on the chuck table 61, the chuck table 61 holding the unit of the optical device wafer 2 and the transfer substrate 3 is moved to a processing start position below the focusing means 622 of the laser beam applying means 62 as shown in FIG. 6A. At this processing start position, one end (left end as viewed in FIG. 6A) of a predetermined one of the streets 23 extending in the first direction is positioned directly below the focusing means 622 of the laser beam applying means 62 as shown in FIG. 6A. Thereafter, a pulsed laser beam having an absorption wavelength to the epitaxy substrate 20 is applied from the focusing means 622 to the epitaxy substrate 20, and the chuck table 61 is moved in the direction shown by an arrow X1 in FIG. 6A at a predetermined feed speed. When the other end (right end as viewed in FIG. 6B) of the predetermined street 23 reaches the position directly below the focusing means 622 of the laser beam applying means 62 as shown in FIG. 6B, the application of the pulsed laser beam is stopped and the movement of the chuck table 61 is also stopped (laser beam applying step). In this laser beam applying step, the focal point P of the pulsed laser beam is set near the back side 20 b (upper surface) of the epitaxy substrate 20.

For example, the laser beam applying step mentioned above is performed under the following processing conditions.

Light source: YAG pulsed laser

Wavelength: 355 nm

Average power: 3.5 W

Repetition frequency: 10 kHz

Focused spot diameter: ellipse having a minor axis set to 10 μm and a major axis set to 10 to 200 μm

Work feed speed: 100 mm/s

The laser beam applying step is repeated two times along the predetermined street 23 under the above processing conditions. As a result, the epitaxy substrate 20 constituting the optical device wafer 2 is cut along the predetermined street 23 extending in the first direction to form a division groove 201 as a laser processed groove having a depth reaching the buffer layer 22 as shown in FIG. 6C (epitaxy substrate dividing step). The epitaxy substrate dividing step is performed along predetermined ones of the plural crossing streets 23 extending in the first direction and the second direction perpendicular to the first direction to thereby divide the epitaxy substrate 20 into a plurality of blocks 200 as shown in FIG. 6D.

By performing the epitaxy substrate dividing step to cut the epitaxy substrate 20 of the optical device wafer 2 along the predetermined streets 23, it is possible to relieve the warpage occurring in the unit of the epitaxy substrate 20 and the transfer substrate 3 due to the difference in coefficient of linear expansion between the epitaxy substrate 20 and the transfer substrate 3.

After performing the epitaxy substrate dividing step along the predetermined streets 23, an optical device wafer supporting step is performed in such a manner that the unit of the optical device wafer 2 and the transfer substrate 3 is attached to a dicing tape as a supporting member supported to an annular frame. More specifically, as shown in FIG. 7, the transfer substrate 3 bonded to the optical device wafer 2 is attached to the front side (upper surface) of a dicing tape T as a supporting member supported to an annular frame F (supporting member attaching step). Accordingly, the back side 20 b of the epitaxy substrate 20 of the optical device wafer 2 bonded to the transfer substrate 3 attached to the upper surface of the dicing tape T is oriented upward. This optical device wafer supporting step may be performed before performing the epitaxy substrate dividing step.

After performing the optical device wafer supporting step, a peeling laser beam applying step is performed in such a manner that a laser beam having a transmission wavelength to the epitaxy substrate 20 is applied from the back side 20 b of the epitaxy substrate 20 to the optical device wafer 2 in the condition where the focal point of the laser beam is set in the buffer layer 22, thereby decomposing the buffer layer 22. This peeling laser beam applying step is performed by using a laser processing apparatus 7 shown in FIG. 8. The laser processing apparatus 7 shown in FIG. 8 includes a chuck table 71 for holding a workpiece and laser beam applying means 72 for applying a laser beam to the workpiece held on the chuck table 71. The chuck table 71 is so configured as to hold the workpiece under suction. The chuck table 71 is movable in a feeding direction shown by an arrow X in FIG. 8 by feeding means (not shown) and also movable in an indexing direction shown by an arrow Y in FIG. 8 by indexing means (not shown). The laser beam applying means 72 includes a cylindrical casing 721 extending in a substantially horizontal direction and focusing means 722 mounted on the front end of the casing 721 for applying a pulsed laser beam to the workpiece held on the chuck table 71.

The peeling laser beam applying step using the laser processing apparatus 7 will now be described with reference to FIG. 8 and FIGS. 9A to 9C. As shown in FIG. 8, the unit of the optical device wafer 2, the transfer substrate 3, and the dicing tape T bonded together is placed on the chuck table 71 of the laser processing apparatus 7 in the condition where the dicing tape T comes into contact with the upper surface of the chuck table 71. By operating suction means (not shown), the unit of the optical device wafer 2, the transfer substrate 3, and the dicing tape T is held under suction on the chuck table 71. That is, the unit of the optical device wafer 2 and the transfer substrate 3 is held under suction on the chuck table 71 through the dicing tape T. Accordingly, the back side 20 b of the epitaxy substrate 20 of the optical device wafer 2 bonded to the transfer substrate 3 attached to the dicing tape T held on the chuck table 71 is oriented upward. While the annular frame F supporting the dicing tape T is not shown in FIG. 8, the annular frame F is held by any suitable frame holding means provided on the chuck table 71.

Thereafter, the chuck table 71 holding the unit of the optical device wafer 2, the transfer substrate 3, and the dicing tape T under suction is moved to a processing start position below the focusing means 722 of the laser beam applying means 72 as shown in FIG. 9A. At this processing start position, one end (left end as viewed in FIG. 9A) of the epitaxy substrate 20 is positioned directly below the focusing means 722 of the laser beam applying means 72 as shown in FIG. 9A. Thereafter, the focal point P of the pulsed laser beam to be applied from the focusing means 722 is set in the buffer layer 22 as shown in FIG. 9B. Thereafter, the laser beam applying means 72 is operated to apply the pulsed laser beam from the focusing means 722 to the buffer layer 22, and the chuck table 71 is moved in the direction shown by an arrow X1 in FIG. 9A at a predetermined feed speed. When the other end (right end as viewed in FIG. 9C) of the epitaxy substrate 20 reaches the position directly below the focusing means 722 as shown in FIG. 9C, the application of the pulsed laser beam is stopped and the movement of the chuck table 71 is also stopped (peeling laser beam applying step). This peeling laser beam applying step is performed over the entire surface of the buffer layer 22. As a result, the buffer layer 22 is decomposed to lose its binding function of binding the epitaxy substrate 20 and the optical device layer 21.

For example, the peeling laser beam applying step mentioned above is performed under the following processing conditions.

Light source: excimer pulsed laser

Wavelength: 284 nm

Average power: 0.08 W

Repetition frequency: 50 kHz

Focused spot diameter: 400 μm

Work feed speed: 20 mm/s

In the epitaxy substrate dividing step mentioned above, the epitaxy substrate 20 is cut along the predetermined streets 23 into the plural blocks 200 to thereby relieve the warpage occurring in the unit of the epitaxy substrate 20 and the transfer substrate 3 due to the difference in coefficient of linear expansion between the epitaxy substrate 20 and the transfer substrate 3. Accordingly, in performing the peeling laser beam applying step, the focal point P of the pulsed laser beam to be applied from the focusing means 722 can be accurately positioned in the buffer layer 22. Accordingly, it is possible to eliminate the problem such that the focal point of the pulsed laser beam may be set in the optical device layer 21 to cause damage to the optical device layer 21. The buffer layer 22 is formed of gallium nitride (GaN), and it is decomposed as 2GaN→2Ga+N2 by the application of the laser beam. Thus, N2 gas is produced to have an adverse effect on the optical device layer 21. However, since the epitaxy substrate 20 is divided along the division grooves 201 into the plural blocks 200, the N2 gas produced can be relieved through the division grooves 201 to thereby reduce the adverse effect on the optical device layer 21.

After performing the peeling laser beam applying step, an epitaxy substrate peeling step is performed in such a manner that the epitaxy substrate 20 divided into the plural blocks 200 is peeled off from the optical device layer 21. More specifically, by performing the peeling laser beam applying step, the binding function of the buffer layer 22 binding the epitaxy substrate 20 and the optical device layer 21 is lost. Accordingly, the plural blocks 200 of the epitaxy substrate 20 can be easily peeled off from the optical device layer 21 as shown in FIG. 10. By peeling off the epitaxy substrate 20 of the optical device wafer 2 from the optical device layer 21 after performing the peeling laser beam applying step as mentioned above, the optical device layer 21 formed on the surface of the epitaxy substrate 20 can be smoothly transferred to the transfer substrate 3.

After performing the epitaxy substrate peeling step to transfer the optical device layer 21 formed on the surface of the epitaxy substrate 20 of the optical device wafer 2 to the transfer substrate 3, a transfer substrate dividing step is performed in such a manner that the transfer substrate 3 is cut together with the optical device layer 21 along the streets 23. A first preferred embodiment of the transfer substrate dividing step will now be described with reference to FIG. 11 and FIGS. 12A to 12D. The first preferred embodiment of the transfer substrate dividing step may be performed by using the cutting apparatus 5 shown in FIG. 3. The transfer substrate dividing step using the cutting apparatus 5 shown in FIG. 3 is performed in the following manner. The unit of the optical device layer 21, the transfer substrate 3, and the dicing tape T bonded together is placed on the chuck table 51 of the cutting apparatus 5 in the condition where the dicing tape T comes into contact with the upper surface of the chuck table 51 as shown in FIG. 11. By operating suction means (not shown), the unit of the optical device layer 21, the transfer substrate 3, and the dicing tape T is held under suction on the chuck table 51. That is, the unit of the optical device layer 21 and the transfer substrate 3 is held under suction on the chuck table 51 through the dicing tape T. Accordingly, the optical device layer 21 bonded to the transfer substrate 3 attached to the dicing tape T is oriented upward. While the annular frame F supporting the dicing tape T is not shown in FIG. 11, the annular frame F is held by any suitable frame holding means provided on the chuck table 51. The chuck table 51 thus holding the unit of the optical device layer 21, the transfer substrate 3, and the dicing tape T under suction is moved to a position directly below the imaging means 53 by the feeding means (not shown).

When the chuck table 51 is positioned directly below the imaging means 53, an alignment operation is performed by the imaging means 53 and the control means (not shown) to detect a cutting area of the transfer substrate 3 to which the optical device layer 21 is bonded. The imaging means 53 and the control means (not shown) perform image processing such as pattern matching for making the alignment between the cutting blade 521 and the streets 23 extending in the first direction on the optical device layer 21 bonded to the transfer substrate 3, thereby performing the alignment in the cutting area for the streets 23 extending in the first direction (alignment step). Similarly, the imaging means 53 and the control means perform the alignment in the cutting area for the other streets 23 extending in the second direction perpendicular to the first direction on the optical device layer 21 bonded to the transfer substrate 3.

After performing the alignment operation for detecting the cutting area of the unit of the optical device layer 21 and the transfer substrate 3 held on the chuck table 51, the chuck table 51 holding the unit of the optical device layer 21 and the transfer substrate 3 bonded together is moved to a cutting start position below the cutting blade 521 as shown in FIG. 12A. At this cutting start position, one end (left end as viewed in FIG. 12A) of a predetermined one of the streets 23 extending in the first direction is positioned on the right side of the cutting blade 521 by a predetermined small amount as shown in FIG. 12A. Thereafter, the cutting blade 521 is rotated in the direction shown by an arrow 521 a in FIG. 12A and simultaneously moved down by a predetermined amount in the direction shown by an arrow Z1 in FIG. 12A from the standby position shown by a phantom line in FIG. 12A to the working position shown by a solid line in FIG. 12A. This movement from the standby position to the working position is performed by operating the vertically moving means (not shown) as mentioned above. The working position of the cutting blade 521 is set so that the outer circumference of the cutting blade 521 reaches the dicing tape T. After moving the cutting blade 521 from the standby position to the working position as mentioned above, the chuck table 51 is moved at a predetermined feed speed in the direction shown by an arrow X1 in FIG. 12A as rotating the cutting blade 521 in the direction shown by the arrow 521 a. When the other end (right end as viewed in FIG. 12B) of the predetermined street 23 extending in the first direction reaches a position on the left side of the cutting blade 521 by a predetermined small amount as shown in FIG. 12B, the movement of the chuck table 51 is stopped. Thereafter, the cutting blade 521 is raised to the standby position shown by a phantom line in FIG. 12B in the direction shown by an arrow Z2 in FIG. 12B.

As a result, the transfer substrate 3 and the optical device layer 21 bonded thereto are cut along the predetermined street 23 extending the first direction to form a division groove 31 as a cut groove as shown in FIG. 12C (transfer substrate dividing step). The transfer substrate dividing step is performed along all of the crossing streets 23 extending in the first direction and the second direction perpendicular to the first direction to thereby form a plurality of crossing division grooves 31 along all of the crossing streets 23 as shown in FIG. 12D. That is, the unit of the optical device layer 21 and the transfer substrate 3 bonded together is divided along all of the crossing streets 23 into the individual optical devices 24 respectively bonded to a plurality of pieces 30 divided from the transfer substrate 3.

A second preferred embodiment of the transfer substrate dividing step will now be described with reference to FIG. 13 and FIGS. 14A to 14D. The second preferred embodiment of the transfer substrate dividing step may be performed by using the laser processing apparatus 6 shown in FIG. 5. The transfer substrate dividing step using the laser processing apparatus 6 shown in FIG. 5 is performed in the following manner. The unit of the optical device layer 21, the transfer substrate 3, and the dicing tape T bonded together is placed on the chuck table 61 of the laser processing apparatus 6 in the condition where the dicing tape T comes into contact with the upper surface of the chuck table 61 as shown in FIG. 13. By operating suction means (not shown), the unit of the optical device layer 21, the transfer substrate 3, and the dicing tape T is held under suction on the chuck table 61. That is, the unit of the optical device layer 21 and the transfer substrate 3 is held under suction on the chuck table 61 through the dicing tape T. Accordingly, the optical device layer 21 bonded to the transfer substrate 3 attached to the dicing tape T is oriented upward. While the annular frame F supporting the dicing tape T is not shown in FIG. 13, the annular frame F is held by any suitable frame holding means provided on the chuck table 61. The chuck table 61 thus holding the unit of the optical device layer 21, the transfer substrate 3, and the dicing tape T under suction is moved to a position directly below the imaging means 63 by the feeding means (not shown).

When the chuck table 61 is positioned directly below the imaging means 63, an alignment operation is performed by the imaging means 63 and the control means (not shown) to detect a processing area of the transfer substrate 3 to be laser-processed to which the optical device layer 21. The imaging means 63 and the control means (not shown) perform image processing such as pattern matching for making the alignment between the streets 23 extending in the first direction on the optical device layer 21 bonded to the transfer substrate 3 and the focusing means 622 of the laser beam applying means 62 for applying a laser beam along the streets 23, thereby performing the alignment in the processing area for the streets 23 extending in the first direction (alignment step). Similarly, the imaging means 63 and the control means perform the alignment in the processing area for the other streets 23 extending in the second direction perpendicular to the first direction on the optical device layer 21 bonded to the transfer substrate 3.

After performing the alignment operation for detecting the processing area of the unit of the optical device layer 21 and the transfer substrate 3 held on the chuck table 61, the chuck table 61 holding the unit of the optical device layer 21 and the transfer substrate 3 bonded together is moved to a processing start position below the focusing means 622 of the laser beam applying means 62 as shown in FIG. 14A. At this processing start position, one end (left end as viewed in FIG. 14A) of a predetermined one of the streets 23 extending in the first direction is positioned directly below the focusing means 622 of the laser beam applying means 62 as shown in FIG. 14A. Thereafter, a pulsed laser beam having an absorption wavelength to the transfer substrate 3 is applied from the focusing means 622 to the transfer substrate 3, and the chuck table 61 is moved in the direction shown by an arrow X1 in FIG. 14A at a predetermined feed speed. When the other end (right end as viewed in FIG. 14B) of the predetermined street 23 reaches the position directly below the focusing means 622 as shown in FIG. 14B, the application of the pulsed laser beam is stopped and the movement of the chuck table 61 is also stopped (laser beam applying step). In this laser beam applying step, the focal point P of the pulsed laser beam is set near the upper surface of the optical device layer 21 bonded to the transfer substrate 3.

For example, the laser beam applying step mentioned above is performed under the following processing conditions.

Light source: YAG pulsed laser

Wavelength: 355 nm

Average power: 7 W

Repetition frequency: 10 kHz

Focused spot diameter: ellipse having a minor axis set to 10 μm and a major axis set to 10 to 200 μm

Work feed speed: 100 mm/s

The laser beam applying step is repeated four to six times along the predetermined street 23 under the above processing conditions. As a result, the transfer substrate 3 and the optical device layer 21 bonded thereto are cut along the predetermined street 23 extending in the first direction to form a division groove 31 as a laser processed groove as shown in FIG. 14C (transfer substrate dividing step). The transfer substrate dividing step is performed along all of the crossing streets 23 extending in the first direction and the second direction perpendicular to the first direction to thereby form a plurality of crossing division grooves 31 along all of the crossing streets 23 as shown in FIG. 14D. That is, the unit of the optical device layer 21 and the transfer substrate 3 bonded together is divided along all of the crossing streets 23 into the individual optical devices 24 respectively bonded to a plurality of pieces 30 divided from the transfer substrate 3.

The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention. 

What is claimed is:
 1. An optical device wafer processing method for transferring an optical device layer in an optical device wafer to a transfer substrate, said optical device layer being formed on the front side of an epitaxy substrate through a buffer layer, said optical device layer being partitioned by a plurality of crossing streets to define a plurality of regions where a plurality of optical devices are respectively formed, said optical device wafer processing method comprising: a transfer substrate bonding step of bonding said transfer substrate to the front side of said optical device layer formed on the front side of said epitaxy substrate through said buffer layer; an epitaxy substrate dividing step of cutting said epitaxy substrate along predetermined ones of said plurality of crossing streets into a plurality of blocks after performing said transfer substrate bonding step; a peeling laser beam applying step of applying a laser beam having a transmission wavelength to said epitaxy substrate from the back side of said epitaxy substrate to a unit of said optical device wafer and said transfer substrate in a condition where the focal point of said laser beam is set in said buffer layer after performing said epitaxy substrate dividing step, thereby decomposing said buffer layer; and an epitaxy substrate peeling step of peeling off said epitaxy substrate divided into said plurality of blocks from said optical device layer after performing said peeling laser beam applying step.
 2. The optical device wafer processing method according to claim 1, wherein said epitaxy substrate dividing step comprises the step of cutting said epitaxy substrate along said predetermined streets by using a cutting blade.
 3. The optical device wafer processing method according to claim 1, wherein said epitaxy substrate dividing step comprises the step of cutting said epitaxy substrate along said predetermined streets by applying a laser beam to said epitaxy substrate along said predetermined streets.
 4. The optical device wafer processing method according to claim 1, further comprising a transfer substrate dividing step of cutting said transfer substrate together with said optical device layer along said plurality of crossing streets after performing said epitaxy substrate peeling step. 